A ceramic sintered body has properties of light weight, high hardness, high wear resistance, high anti-corrosion resistance and low thermal expansion coefficient, so that the ceramic sintered body has been widely used as a member for constituting precious devices. In particular, from the viewpoint of excellence in high hardness and the wear resistance, the ceramic sintered body has been preferably utilized as the wear resistant member constituting a bearing. Among various ceramic sintered bodies, a silicon nitride (Si3N4) sintered body has a high hardness and an excellent wear resistance, so that the silicon nitride sintered body has been preferably used as a member for constituting the bearings or the like.
Regarding this silicon nitride sintered body, for the purpose of improving a reliability as the wear resistant member for constituting the bearings or the like, further improvements in properties have been advanced in these days. For example, there has been proposed a method of manufacturing the silicon nitride sintered body, comprising the steps of:
preparing a material powder mixture by adding yttrium oxide, spinel, aluminum oxide and/or aluminum nitride to a silicon nitride material powder at predetermined molar ratio of specified metal elements and predetermined content ratio;
preparing a compact body (molded body) formed from the material powder mixture;
sintering the compact body at a temperature of 1400 to 1500° C.;
further sintering the compact body at a temperature of 1500 to 1650° C. in a nitrogen gas atmosphere of 10 atm or higher thereby to obtain a sintered body having a relative density of about 98%; and
conducting a secondary sintering operation to the sintered body at a temperature of 1400 to 1650° C. thereby to increase the relative density to be over 98%, so that there can be manufactured a silicon nitride sintered body excellent in strength and having less scattering in strength properties (for example, refer to Patent Document 1).
Further, as the silicon nitride raw material powder used for manufacturing this type of the silicon nitride sintered body, it is generally known that a high-purity material powder is preferably used. for example, a high purity material powder synthesized through an imido thermal decomposition method has been suitably used.
However, this high-purity material powder is costly expensive, and mechanical strength and fracture toughness values of the manufactured silicon nitride sintered body are tend to become excessively large, so that there is posed a problem such that a workability of the resultant sintered body is not sufficient.
In order to solve this problem, there has been reviewed a method of manufacturing the silicon nitride sintered body by using a cheap silicon nitride raw material powder manufactured through a direct nitriding method in which a metal Si is directly nitrided. The silicon nitride raw material powder manufactured through the direct nitriding method has relatively large Fe and Ca contents.
However, it is know that when the contents of the rare earth element, aluminum component and silicon carbide or the like are controlled to be within predetermined ranges, so that there can be obtained a sintered body having a mechanical strength, a wear resistance and a rolling life property that are equal to or more than those of conventional sintered bodies, and the resultant sintered body is also excellent in workability (for example, refer to Patent Document 2).
Further, there has been reviewed a method for suppressing the sintered body from generating defects by lowering a total oxygen content at peripheral portion of the sintered body whereby a depth of the defects such as pore and crack can become shallow, and a generation of failure due to these defects can be effectively suppressed (for example, refer to Patent Document 3).    Patent Document 1: Japanese Patent Application (Publication) No. 06-080470 (Examples or the Like)    Patent Document 2: International Patent Application (Publication) No. 2005/030674    Patent Document 3: Japanese Patent Application (Publication) No. 2002-326875